Interaction of [D-Ser2,Leu5]enkephalin-Thr6 (DSLET), a relatively selective delta ligand, with mu1 opioid binding sites.

Using binding approaches, we have confirmed the high selectivity of [D-Ser2,Leu5]enkephalin-Thr6 (DSLET) to delta, as opposed to morphine-preferring (mu2) sites in rat brain. However, detailed experiments studies indicate that this ligand also labels mu1 sites with very high affinity. Saturation studies of 3H-DSLET binding reveal curvilinear plots. Treating tissue with naloxonazine to block mu1 sites, eliminates the higher affinity binding component. Competition studies of the other peptides against 3H-DSLET and 3H[D-Ala2,MePhe4,Gly(ol)5]enkephalin (3H-DAMPGO) binding also implied high affinity binding of these peptides to mu1 sites. The ability of these peptides to interact with mu1 sites may help explain some of their pharmacological actions.